The present invention relates to a process and apparatus for the removal and causticisation of sodium oxalate and/or sodium sulphate from a Bayer process liquor.
In the Bayer process for the production of alumina bauxite is digested in a caustic liquor, generally under conditions of elevated temperature and pressure. A variety of organic and inorganic impurities are invariably extracted at the same time, reacting with caustic soda to form their sodium salts. In addition, some of the organic compounds can undergo degradation, ultimately producing sodium carbonate and the sodium salts of a range of simple carboxylic acids. The formation of these impurities represents a major loss of caustic from the refinery""s liquor streams. This caustic must either be replaced, or recovered in some way from the impurities.
The recovery of caustic from sodium carbonate is a commonplace activity in most alumina refineries. The causticisation of sodium carbonate is generally effected by the addition of lime, which reacts with the sodium carbonate to form calcium carbonate, thereby liberating sodium hydroxide. An improved version of this process is described in our co-pending International Application No. PCT/AU99/00757, filed on the 25th of Sep. 1999 and entitled xe2x80x9cImproved Bayer Causticisationxe2x80x9d, and co-pending U.S. patent application Ser. No. 09/787,943, filed Mar. 23, 2001 and which is the National Stage of International Application No. PCT/AU99/00757. The contents of PCT/AU99/00757 and 09/787,943 are incorporated herein by reference.
Of the other impurities, sodium oxalate and sodium sulphate are among the most significant The presence of sodium oxalate in Bayer process steams is problematical owing to its very limited solubility. This creates a number of well-known problems within the alumina refinery. Sodium sulphate is much more soluble, and can accumulate to very high concentrations. This causes a different set of problems, particularly with respect to the refinery""s productivity. The problems associated with this impurity in Bayer process liquors, and a process for its separation, have been described in Australian patent No. 673306.
Many prior art processes have been described for the removal of sodium oxalate and sodium sulphate from Bayer liquors. Some of these processes remove both impurities concurrently. In most cases, these processes advocate that the impurity is discarded after removal from the liquor stream. However, a small number of the above processes also provide a means for the recovery of soda from sodium oxalate. None describe a practical method for the recovery of soda from sodium sulphate, requiring that it be discarded. However, disposal of sodium sulphate is not straightforward.
Environmental considerations preclude disposal of sodium sulphate into natural water systems, and since it is highly soluble, it must be disposed in a suitably lined or otherwise isolated sanitary landfill if it is not to enter groundwater systems. In the alumina refinery, disposal of sodium sulphate to the red mud residue disposal areas results in the eventual return of most of the sodium sulphate to the process liquor stream with the recovered lake water.
Whilst it is preferable to utilise the sodium sulphate in some way, for example by conversion into useful products, options for this are extremely limited. Electrolytic cells are commercially available which convert sodium sulphate into sodium hydroxide and either sodium bisulphate or sulphuric acid. However, these are generally restricted to reasonably pure solutions in which scales are unlikely to form, because the membranes used in the cells are sensitive to fouling. Other processes have been investigated including reductive processes such as the Leblanc process, and the Peniakoff process for production of gibbsite from bauxite. These latter processes are not currently practised, as they are inefficient, costly and produce eavironmentally unacceptable by-products.
Thus, there is a significant need for an economic process for the processing of sodium sulphate into more useful products, and/or for the immobilisation of the sulphate anion in an environmentally acceptable, insoluble material.
Most alumina refineries practice some form of oxalate removal process. In general, these processes are based on variations of the following two procedures:
1. Sodium oxalate is permitted to coprecipitate with gibbsite in the refinery""s gibbsite precipitation circuit. The co-crystallised oxalate reports to the refinery""s gibbsite seed preparation facility, where it is removed by washing with water or dilute liquor. The oxalate-rich washings are then further treated to remove oxalate either by seeding and evaporation to recrystallise sodium oxalate or, by reaction with lime, as calcium oxalate.
2. Oxalate co-crystallisation is avoided by crystallising and removing sodium oxalate in a side-stream of one of the refinery""s main process streams (usually a spent liquor stream). The side stream is evaporated to increase the supersaturation of the sodium oxalate and directed to a series of oxalate crystallisers where it is seeded with recycled sodium oxalate crystals. After solid/liquid separation, the clarified and now oxalate-depleted liquor is returned to the process. A portion of the solid sodium oxalate is recycled to act as seed, while the remainder is either discarded or processed to recover soda. An example of this process is outlined in U.S. Pat. No. 3,899,571.
Most processes for the recovery of the soda values from sodium oxalate are based on reactions with lime. In some processes, the separated sodium oxalate cake is fist burnt in a kiln to produce sodium carbonate, which is subsequently causticised by reaction with lime. This process is costly to operate, and the conversion to sodium carbonate is not always complete.
In other processes, a solution rich in sodium oxalate, such as the washings from the seed circuit of a refinery that practices coprecipitation of oxalate, is directly reacted with lime to form calcium oxalate. However, whilst very low oxalate concentrations can be achieved in the ted stream in this way, the efficiency of lime utilisation is very poor, due to the formation of calcium aluminates such as tricalcium aluminate (TCA), unless the stream is very low in caustic and sodium aluminate. Consequently, this process can only be applied to dilute liquors.
The present invention was developed with a view to providing a means for the direct removal of sodium sulphate or sodium oxalate, or combinations of both, in Bayer process liquors with the production of sodium hydroxide. The unwanted anion is isolated in an insoluble solid material that can be disposed of in a conventional sanitary landfill, thus preventing the return of the unwanted anions to the refinery via the refinery""s lake system.
Throughout this specification, we have used conventional North American terminology for the description of Bayer solution compositions. Thus, xe2x80x98Cxe2x80x99 refers to the caustic concentration of the liquor, this being the sum of the sodium aluminate and sodium hydroxide content of the liquor expressed as equivalent g/L of sodium carbonate. xe2x80x98Sxe2x80x99 refers to the sum of C and the true concentration of sodium carbonate. Thus, S-C gives the actual concentration of Na2CO3 in the liquor, in g/L. xe2x80x98Axe2x80x99 refers to the concentration of sodium aluminate in the liquor, expressed as equivalent g/L of Al2O3.
Sodium oxalate concentration is expressed as g/L of Na2C2O4. Sodium sulphate concentration is expressed as g/L of Na2SO4. xe2x80x98TSxe2x80x99 refers to the sum of all sodium salts in solution, expressed as the equivalent concentration in g/L of sodium carbonate.
Lime refers either to calcium oxide, or more preferably, calcium hydroxide. Lime efficiency is defined as the percentage ratio of the number of moles of sodium hydroxide produced to the number of moles of lime consumed, divided by two.
The term Hydrocalumite is used to refer to any layered double hydroxide compound formed between calcium and aluminium, within which charge balancing anions are intercalated. Typically, these compounds will be of the form [Ca2Al(OH)6]2.X.nH2O, where xe2x80x98Xxe2x80x99 represents a charge-balancing anion or anions.
Throughout this specification the term xe2x80x9ccomprisingxe2x80x9d is used inclusively, in the sense that there may be other features and/or steps included in the invention not expressly defined or comprehended in the features or steps subsequently defined or described. What such other features and/or steps may include will be apparent from the specification read as a whole.
According to one aspect of the present invention there is provided a process for the removal and causticisation of sodium oxalate and/or sodium sulphate from a Bayer process liquor containing sodium carbonate and one or both of sodium oxalate and sodium sulphate in an alumina refinery, the process comprising the steps of:
removing aluminate ions from the Bayer liquor through the formation of carbonate-bearing hydrocalumite and/or sulphate-bearing hydrocalumite; and,
treating the liquor with sufficient lime to remove and causticise any residual carbonate ions and some or all of the oxalate ions present whereby any reacted lime solids thus formed can be separated and safely disposed of.
Preferably the process comprises a further step, prior to said step of removing aluminate ions, in which the liquor is enriched with sulphate and/or oxalate such that any aluminate and/or carbonate ions entering with the sulphate and/or oxalate are also removed.
Preferably the process comprises a further step, following said step of removing aluminate ions, of separating the carbonate-bearing hydrocalumite and/or sulphate-bearing hydrocalumite from the Bayer liquor to form a clarified liquor.
In another embodiment the process further comprises a pre-causticisation step in which the Bayer liquor is first causticised to reduce the concentration of carbonate ions, prior to said step of removing aluminate ions. Typically said pre-causticisation step includes heating the liquor, adding sufficient lime to react with the carbonate ions to produce calcium carbonate and separating the calcium carbonate from the liquor. Optionally the heated liquor is enriched with sulphate and/or oxalate prior to causticisation to ensure that any carbonate ions entering with the sulphate and/or oxalate are also causticised.
According to another aspect of the present invention there is provided an apparatus for the removal and causticisation of sodium oxalate and/or sodium sulphate from a Bayer process liquor containing sodium carbonate and one or both of sodium oxalate and sodium sulphate in an alumina refinery, the apparatus comprising:
means for removing aluminate ions from the Bayer liquor through the formation of a carbonate-bearing hydrocalumite and/or sulphate-bearing hydrocalumite; and,
means for treating the liquor with sufficient lime to remove and causticise any residual carbonate ions and some or all of the oxalate ions present whereby any reacted lime solids thus formed can be separated and safely disposed of.
In one embodiment said means for removing aluminate ions comprises a first reaction vessel to which sufficient lime is added to react with all of the aluminate ions in the liquor. Preferably the apparatus of this embodiment further comprises a means for separating the carbonate-bearing hydrocalumite and/or sulphate-bearing hydrocalumite from the liquor to form a clarified liquor. Preferably the means for treating the liquor comprise a second reaction vessel to which sufficient lime is added to react with the sodium oxalate in the clarified liquor to form calcium oxalate and with any remaining carbonate ions to form sodium carbonate.
In another embodiment said means for removing aluminate ions and said means for treating the liquor are comprised in a single reaction vessel to which sufficient lime is added to react with the aluminate ions to form said carbonate-bearing and/or sulphate-bearing hydrocalumite, together with sufficient additional lime to react with the sodium oxalate to form calcium oxalate.
The inventors"" theories on the chemical reactions in the and apple of the present invention are merely examples of possible reactions thought to be taking place and are not intended to be Uniting in any way.